Light photons, color and energies of molecules.

In summary, the conversation discussed the topic of light absorption by molecules and its relation to color perception. It was explained that when a photon interacts with a molecule, its energy is transferred to the molecule and the photon is essentially absorbed. The length of the molecule's conjugate chain affects the wavelength of the photon that gets absorbed. It was also discussed that in the absence of light, objects do not have a color since color is perceived by the brain through light interaction. The conversation also touched on the topic of how molecules return to their electronic ground state after absorbing a photon and how this can lead to heat or even a change in color in some cases.
  • #1
Joey V
4
0
Hi, so I'm a first year neuroscience student at Carelton University in Canada. I had a little bit of a "revelation" with this topic recently after I understood it a bit better and I think this is really interesting. (If I understand it correctly!) We're learning about Kekule structures, conjugation (alternating single/double bonds in a molecule) and color.

So basically what I understand is that, obviously, every wavelength of light has a different energy associated with it. This is the same for light photons. So when a light photon interacts with a molecule, it transfers its energy to the molecule and the photon is essentially "destroyed" or "absorbed". The length of the conjugate chain in the molecule plays a part in determining the wavelength of photon that gets absorbed by that molecule. So depending on how long the chain is, the difference in energy between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital is affected.

What I understand is that when a photon (Let's pretend it's green) has roughly the same energy as the gap between the HOMO and LUMO, that photon will be absorbed by that molecule. This is because as the energy associated with the green photon bumps into an electron from the HOMO orbital and gives it energy to sit in the next orbital above it, LUMO. Now that the green photon just got absorbed we see all the other photons (or wavelengths of light) being reflected off of the object and we notice that there is no more green light being reflected off of the object. AKA we perceive that as "every color except green" or blue. (I think it's blue anyway).

Now this was the main part of my question:
If we turn off the lights in a room so that there is no visible light at all, do the objects that were originally absorbing green photons lose energy and have electrons drop back down to the HOMO orbital and lose their color? Essentially does this mean that in the dark, objects physically/chemically change and lose their color? Is this why we perceive shadows as being black, because an object is blocking photons from reaching the ground and since no (or less) photons are hitting the ground, it essentially has no color?

This was quite a doozy to write, thanks a lot for reading this and any answers, opinions or further questions would be greatly appreciated. I can't find anything on this on the internet so I'm going to ask my chemistry professor after the weekend and I'll update this thread after I get an answer from him.

Thanks again
 
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  • #2
Joey V said:
If we turn off the lights in a room so that there is no visible light at all, do the objects that were originally absorbing green photons lose energy and have electrons drop back down to the HOMO orbital
Yes.

Joey V said:
and lose their color?
If a tree falls in the forest... Colour is something perceived / interpreted by our visual system, stemming from the interaction of light with objects. If there is no light, it doesn't make sense to talk about colour. But I wouldn't call that "losing" a colour, as the same object will appear the same colour as soon as light is shone upon it again.

Joey V said:
Essentially does this mean that in the dark, objects physically/chemically change and lose their color?
The objects will not be in the same physical state, but in most cases there will not be any chemical change. That said, light can induce chemical changes, and in some cases lead to an actual (i.e., permanent) change in colour. Bleaching is one such phenomenon.

Joey V said:
Is this why we perceive shadows as being black, because an object is blocking photons from reaching the ground and since no (or less) photons are hitting the ground, it essentially has no color?
When there is no light, there is no stimulation of the visual system, and our brain interprets this as black. That is, as far as I understand this, but you are the one studying neuroscience!
 
  • #3
DrClaude said:
Yes.If a tree falls in the forest... Colour is something perceived / interpreted by our visual system, stemming from the interaction of light with objects. If there is no light, it doesn't make sense to talk about colour. But I wouldn't call that "losing" a colour, as the same object will appear the same colour as soon as light is shone upon it again.The objects will not be in the same physical state, but in most cases there will not be any chemical change. That said, light can induce chemical changes, and in some cases lead to an actual (i.e., permanent) change in colour. Bleaching is one such phenomenon.When there is no light, there is no stimulation of the visual system, and our brain interprets this as black. That is, as far as I understand this, but you are the one studying neuroscience!

Thanks for the answer, makes sense to me. And of course I understand that color is perceived by our brain and such, but I was thinking about this in more of a "particle interaction" sense. I also like the tree falling in the forest analogy too!

I'll still be asking my chemistry professor to see what he says about this, but I get what you're saying, it's definitely more clear now.
 
  • #4
I guess you need still to understand what happens to a molecule after absorption of a photon. Usually, the electronic degrees of freedom are strongly coupled to vibrational degrees of freedom of the molecule. Hence the electronically excited states decay on a sub-picosecond scale back to the electronic ground state with the molecule strongly vibrating (this is called a radiationless transition). Vibrations of a molecule is nothing else than heat, so that is the basic mechanism how substances heat up when left in the sun.
So the molecule is basically all of the time in it's electronic ground state.
There are only few exceptions: One are fluorescent materials where the coupling of the electronic and vibrational degrees of freedom is small. Hence these molecules can emit a photon when falling back to the ground state.
This process is also very rapid.
The other exception are phosphorescent materials. There the coupling of the excited electronic state to the ground state is very small, so that neither vibrational nor radiational processes are fast, and the radiation emitted when falling back to the ground state can be observed minutes or even hours after exitation. You certainly remember those gowing stars from your craddle.
 
  • #5
for your question and interest in this topic! It's great to hear that you had a "revelation" with this topic and are eager to learn more about it. I can definitely say that understanding the interactions between light photons, color, and energies of molecules is a fascinating and important area of study.

You are correct in your understanding that different wavelengths of light have different energies associated with them. This is due to the fact that light is made up of particles called photons, which have both wave-like and particle-like properties. When a photon interacts with a molecule, it can transfer its energy to the molecule, causing an electron to jump to a higher energy level. This energy transfer is what gives molecules their color.

The length of the conjugate chain in a molecule does play a role in determining the wavelength of light that gets absorbed. This is because the length of the chain affects the energy difference between the HOMO and LUMO levels. As you mentioned, when a photon with the same energy as this difference is absorbed, it causes an electron to jump to the LUMO level, resulting in the molecule appearing a certain color to us.

To address your question about objects losing their color in the dark, it's important to understand that the color of an object is determined by the wavelengths of light that it reflects or absorbs. When there is no visible light in a room, the objects in that room are not receiving any light photons and therefore cannot absorb or reflect any specific wavelengths. This does not necessarily mean that the objects are losing energy or undergoing any chemical changes. However, if the absence of light is prolonged, it is possible that the molecules in the object may eventually undergo chemical changes that could affect its color.

As for shadows appearing black, this is because a shadow is an area where light is blocked or absorbed by an object, creating a lack of light. Without any light, we are unable to see any color from that area, resulting in the perception of black.

I hope this helps to clarify your understanding of this topic. It's always great to see students making connections and asking insightful questions. Keep up the curiosity and you will continue to have many more "revelations" in your studies!
 

1. What are light photons?

Light photons are particles of electromagnetic radiation that have both energy and momentum. They are the fundamental unit of light and are responsible for carrying energy and transmitting information in the form of electromagnetic waves.

2. How do photons interact with molecules?

Photons can interact with molecules in three ways: absorption, emission, and scattering. In absorption, a photon is absorbed by a molecule, increasing its energy level. In emission, a molecule releases a photon as it returns to its original energy level. In scattering, a photon is deflected by a molecule without being absorbed or emitted.

3. What determines the color of a molecule?

The color of a molecule is determined by the energy of the photons it absorbs or emits. When a molecule absorbs a photon, it gains energy and jumps to a higher energy level. The color we see is the result of the difference in energy between the absorbed and emitted photons.

4. How do the energies of molecules affect their behavior?

The energy of a molecule determines its behavior and properties. Molecules with higher energy levels are more reactive and can undergo chemical reactions more easily. The energy of a molecule also affects its physical properties, such as melting and boiling points.

5. Can the energy of a molecule be changed?

Yes, the energy of a molecule can be changed through various processes, such as absorption, emission, and collision with other molecules. These changes in energy can lead to different behaviors and properties of the molecule.

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